1. Field of the Invention
The present invention relates to a nitride semiconductor device using a silicon substrate and method of manufacturing the nitride semiconductor device, and more particularly, relates to a nitride semiconductor device having an electrode of excellent ohmic contact properties and adhesion with respect to the silicon substrate and method of manufacturing the nitride semiconductor device.
2. Description of the Related Art
A nitride semiconductor device is capable of producing high-output, short-wavelength light and has widely been used for a blue or green LED and for a white LED, in which a fluorescent material is used in combination of a nitride semiconductor device. In addition, a nitride semiconductor device has been intensively studied also as a high-speed electric device such as HEMT.
When manufacturing a nitride semiconductor device, a nitride semiconductor layer is generally grown by hetero-epitaxial growth on a foreign substrate such as sapphire, because a GaN substrate for homo-epitaxial growth of a nitride semiconductor layer is expensive. However, obtaining a wafer having a large diameter is difficult even by using a foreign substrate such as sapphire, nor at a lower cost. Also, since sapphire is insulative, an electrode cannot be formed on the back surface of a sapphire substrate and electrodes of different polarities are needed to be formed at the same side of the nitride semiconductor layer. Thus, a uniform current distribution within the nitride semiconductor device is difficult to obtain.
Therefore, consideration has been made about fabrication of a nitride semiconductor component on a silicon substrate, which is conductive and an electrode can be formed on the back surface thereof, and a wafer having a large diameter is available in the market at a lower price.
For example, described in JP 2002-208729A is a nitride semiconductor device fabricated by forming a metal-compound region having gallium, indium and silicon as its major components and an aluminum nitride layer on an n-type silicon substrate, then forming a nitride semiconductor component structure thereon. On the back surface of the n-type silicon substrate, there is formed a cathode electrode made of vacuum-deposited titanium and aluminum. On the p-side of the nitride semiconductor layer, there is formed an anode electrode made of vacuum-deposited nickel and gold. Also described therein is a formation of an electrode made of sequentially stacked titanium/an alloy of gold, germanium and nickel/gold as substitute for the cathode electrode made of titanium and aluminum.
Described in JP 2003-8061A is a nitride semiconductor device in which a layer containing a IIIB group element is formed on an n-type silicon substrate and then a nitride semiconductor layer is formed thereon. On the back surface of the n-type silicon substrate, there is formed a bonding electrode made of any one of Al, Ti, Zr, Hf, V and Nb. On the p-side of the nitride semiconductor layer, there is formed a thin-film transparent electrode made of any one of Ta, Co, Rh, Ni, Pd, Pt, Cu, Ag and Au, and further formed a second bonding electrode.
Further, described in JP 2005-108863A is a nitride semiconductor device in which a nitride semiconductor layer grown on a foreign substrate such as sapphire is bonded to a silicon substrate via a conductive bonding layer such as an eutectic crystal layer, then the foreign substrate such as sapphire is removed to obtain the nitride semiconductor device.
However, when a nitride semiconductor device is formed by using a silicon substrate as described in JP 2002-208729A, JP 2003-8061A, and JP 2005-108863A, a metal electrode formed on the silicon substrate is easy to detach during the manufacturing process of the device. That is, because a silicon substrate is conductive, it is possible to form a metal electrode on the back surface of the substrate, but the silicon substrate and metal electrode are needed to establish good ohmic contact. However, when a metal electrode is made of a metal having a good ohmic contact with a silicon substrate, the metal electrode may detach from the silicon substrate during the manufacturing process of the nitride semiconductor device such as a dicing process. In a nitride semiconductor device, a nitride semiconductor layer having different thermal expansion coefficient and lattice constant is needed to be formed on a silicon substrate. Therefore, stress loaded on the metal electrode formed on the silicon substrate is larger than that in a silicon semiconductor device. Consequently, it is presumed that detachment of the metal electrode is particularly easy to occur.
Particularly, in the cases where the silicon substrate and the nitride semiconductor layer are coupled by bonding as described in JP 2005-18863A, the growth substrate is removed after bonding three layers of a growth substrate, a nitride semiconductor layer, and a silicon substrate, each having a different thermal expansion coefficient. This causes a significant change in the warpage of the silicon substrate during manufacturing process, resulting in increased occurrence of detachment of the metal electrode. In addition, after bonding the silicon substrate and the nitride semiconductor layer by using a conductive bonding layer as described in JP 2005-18863A, annealing cannot be applied at a higher temperature than the melting point of the conductive bonding layer. Thus, the metal electrode formed on a silicon substrate after bonding needs to be made of a metal capable of establishing ohmic contact without being subjected to annealing at a high temperature, limiting the selection of the electrode materials. Therefore, satisfying both ohmic contact property and adhesion in the metal electrode becomes further difficult.